Various information recording and retrieving systems are known in which a magnetically-sensitive or optically-sensitive tape is made to describe a helical path around the periphery of a structure, usually of cylindrical construction, and including one or more information recording/retrieving devices. Such systems are referred to as helical scan recording systems. The tape may completely encircle the structure so that a tape helix of 360.degree. or more is developed.
The information recording/retrieving devices, which may be magnetic or optical heads, for example, are made to rotate at constant speed and in a fixed plane at right angles to the longitudinal axis of the structure around which the tape is driven. Because of the orientation of the tape relative to the axis of the support structure, a helix angle exists between the rotating information recording/retrieving devices and the direction of tape travel. The angle is governed by the width of the tape and by the diameter of the structure supporting the tape helix. Each information recording/retrieving device scans diagonally across the width of the tape. An information signal fed to the recording/retrieving device is recorded on, and can be reproduced from, a succession of parallel, equal-length tracks, each extending at the same angle across the width of the tape.
In recent years there has been extensive progress in the area of high density recording on light and thermal sensitive tapes. Optical tape media using magneto-optic and reversible crystalline state techniques for erasable recordings, as well as ablative techniques for permanent or archival recordings, are in current use and development.
The recording surface of an optical tape medium includes a coating which is extremely subject to wear. Because of this, the sensitive tape surface cannot be dragged across another surface. As a result, optical tapes are generally caused to "fly" above the peripheral surface of the cylindrical structure, generally referred to as a headwheel, on a stable gas bearing, which generally comprises a compressible fluid, such as air. The bearing may be generated hydrostatically or hydrodynamically. As used in the art the term "hydrostatic" is equivalent to the term "externally-pressurized," and the term "hydrodynamic" is equivalent to the term "self-generated."
Helical-scan tape recording/retrieving systems are subject to peculiar guidance problems, such as tape skew and lateral displacement of the tape. Where a tape is forced out of contact with the headwheel by an air film bearing, the need for accurate tape guidance becomes even more acute. When the conditions of tape skew and lateral displacement do occur, the information recording/retrieving device may be out of tracking position or misaligned with respect to the recorded information tracks. In such a case, the information signal which is retrieved will suffer from low amplitude, or distortion caused from the adjacent data tracks. These undesirable results become severe as track density is increased.
There are many helical tape systems in which the tape, or a portion thereof, is supported on an air bearing where it wraps about the headwheel. Some systems employ hydrostatic forces, some employ hydrodynamic forces, and some use a combination of the two. It will be noted that a headwheel configured to provide a hydrostatic air bearing is more expensive to produce and also requires an external source of pressurized air.
In some helical tape systems, the headwheel is divided into fixed and rotating sections. The rotating portion includes the recording/retrieving device and generally provides a hydrodynamic air bearing, although some systems have implemented hydrostatic support through the rotating portion. The width of the rotating portion is generally determined by the type of force used to generate the bearing. There are systems in which the rotating portion is very narrow, and hydrostatically-generated bearings above and below it provide tape support. In other systems, hydrodynamic support is provided for the top half of the wrap but not for the bottom half, in order to provide tape edge guidance around the headwheel.
At the present time, there is no system known to the applicant which provides hydrodynamic tape support across the entire tape width while providing edge guidance to the tape along the helical wrap.